Hammer protection device for high-speed line printers

ABSTRACT

A protection device for the hammers in a high-speed printer has a comparator which monitors the current flow in all of the hammer drive circuits simultaneously. A logic gate couples the comparator output to a protecting relay when the printer hammers are not being fired.

United States Patent Inventor Norman R. King Bayport, NX.

Aug. 29, 1969 June 29, 1971 Potter Instrument Company,.lnc. Plainview, N.Y.

Appl. No. Filed Patented Assignee HAMMER PROTECTION DEVICE FOR HIGH- SPEED LINE PRINTERS 3 Claims, 1 Drawing Fig.

Int. CL B41j Field of Search 101/93; 340/ I 46.2

[56] References Cited UNITED STATES PATENTS 3,495,216 2/1970 Silverschotz Primary Examiner-Robert Peshock Attorney-Laurence J. Marhoefer ABSTRACT: A protection device for the hammers in a highspeed printer has a comparator which monitors the current flow in all of the hammer drive circuits simultaneously. A logic gate couples the comparator output to a protecting relay when the printer hammers are not being fired.

HAMMER PROTECTION DEVICE FOR HIGI-I-SPEED LINE PRINTERS BACKGROUND OF THE INVENTION The field-of the present invention relates to protecting highspeed printer hammers from excessivecurrents in the event of a malfunction and, more particularly, to inexpensive circuitry for performing this function.

High-speed printers of the type in which this invention may be utilized typically employ between 64 and I28 electromagnetically operated impact hammers which strike individual type faces to print various characters. A solenoid-actuating coil respectively associated with each hammer causes the hammer to move when the coil is energized. In prior art impact printers, each hammer has a circuit interrupter such asja thermally actuated fuse which protects the circuit from excessive current flow in the event of a malfunction. Such fuses are relatively expensive. In addition, because a large current pulse is applied to the hammer coil when the hammer fires and yet only a small current flow for an excessive period is sufficient to damage the hammer, it is difficult to satisfactorily protect the hammer drive coils with prior art devices.

One object of this invention is the provision of a relatively inexpensive, reliable circuit for protecting a hammer in a highspeed impact printer.

SUMMARY OF THE INVENTION In order to achieve the above and other objects, the present invention in one embodiment utilizes a detection circuit which monitors the current flow through all of the, hammer drive coils simultaneously. The output of the detection circuit is gated with a status signal from the printer control such that a fault signal is produced if there is current flowing in one or more of the hammer solenoid coils when the hammers are not firing.

BRIEF DESCRIPTION OF THE DRAWING For a more complete understanding of the invention reference should be made to the drawings wherein the only FIGURE is a partially schematic circuit diagram and partially block diagram of a hammer protection circuit according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the FIGURE, the individual hammers shown as blocks each have a solenoid-operating coil 12. Each coil 12 iscoupled in series between the positive terminal of a power supply 14 and ground by transistor switches 16.

Only four hammer circuits are shown for the sake of clarity; however, it will be appreciated that typically a printer may employ in the neighborhood of to 132 hammers.

As will be understood by those skilled in the high-speed printer art, the hammers l0 serve to impact a paper web 18 and push it and an inked ribbon 22 against a continuously rotating print drum or chain 24. The drum or chain carries raised type faces so that the paper web receives an imprint of the character which it strikes.

The output of a transducer 26, which keeps track of the position of the drum or chain 24 with respect to the hammers 10, is coupled to a printer logic and control circuit 32 of any suitable design well known in the printer art. Another input to the logic and control unit 32 is a data input from a central processor, controller or other suitable data input device. Outputs from the logic unit 32 fire the hammers 10 when a character face corresponding to the data to be printed is in position. To this end, leads 30 couple respectively the printer logic unit 32 to the bases of transistors I6; a current pulse from the printer logic unit 32 switches a transistor 16 rapidly to its low-impedance state, thereby energizing its associated coil 12 and firing hammer 10, When the pulse terminates, transistor 16 switches to its high-impedance state. The printer thus far described is typical of these known in prior art.

In order to prevent the hammers 10 or coils 12 from being damaged in'the event of a malfunction, a solenoid-operated circuit breaker 36 is immediately opened. To this end, diodes 38 respectively couple the output of current-sensing resistors 18 coupled in series with the emitters of transistors 16 to a common or nodal point 42. It will be appreciated that diodes 38 and common point 42 provide a logical OR function. A lead couples common point 42 to one input of a two-input comparator whose other input is coupled to a reference potential source 46. Conveniently, comparator 44 may comprise a differential transistor amplifier with point 44 coupled to the base of the transistor on one side of the amplifier. So long as the potential across all the low-impedance current-sensing resistors 18 is below the potential (7 millivolts, for example) of reference 46, the output of comparator 4 is logically FALSE. When the potential across any resistor 18 rises above the potential of reference 46 the output of amplifier 44 is logically TRUE.

An AND gate 52 couples the output of comparator 44 to the base of a transistor switch 54. A lead 56 couples the other input to gate 52 to the printer logic 32; when both inputs to gate 52 are TRUE, the output of gate 52 turns on switch 54,

opening the circuit breaker 36 and thereby disconnecting the coils 12 from the supply 14. Lead 56 is coupled logically at suitable points in the control unit 32 so that the potential on lead 56 is logically TRUE except when the logic and control unit is in a state to command the firing of hammers 10. When the hammers are firing the potential level on lead 56 is logically FALSE, disenabling gate 54 and thereby preventing the circuit breaker 36 from opening as a result of normal hammer firing current. Thus, if there is current flow through any solenoid-operating coil 12 during an interval when the hammers 10 are not being fired, the circuit breaker opens, protecting the coil and hammer from damage. In printers where each hammer drive circuit is on a separate printed circuit board, a malfunctioning hammer can be quickly located by disconnecting the boards sequentially until output from comparator 44 becomes logically FALSE.

Thus, it will be appreciated by the use of diodes, which are relatively inexpensive as compared with thermal fuses, a sim* ple yet sensitive protection is provided for the print hammers, fulfilling the objects of this invention.

Although the present invention has been described with respect to specific embodiments, it will be appreciated by one skilled in the art that a variety of changes may be made without departing from the scope of the invention. For example, certain features may be used independently of others and equivalents may be substituted all within the spirit and scope of the invention.

What I claim is:

1. In a high-speed impact printer in which a plurality of hammers can fire simultaneously, apparatus for protecting said hammers from excessive current flow comprising, in combination:

a comparator having a signal input means, said comparator producing a characteristic output signal if the signal at said input means exceeds a predetermined level,

means for generating a signal level proportional to the current flow respectively in each of said hammers,

means for coupling said comparator input means to each of said generating means,

a power supply,

means for coupling said power supply to said print hammers to energize said print hammers,

said power supply coupling means including means for decoupling said power supply from said print hammers in response to said characteristic output signal,

said predetermined signal level being substantially less than the signal level generated by said generating means when a print hammer is energized, and

means preventing said uncoupling of said power supply from said print hammers during that portion of a print cycle during which said print hammers are firing.

hammers can fire simultaneously, apparatus for protecting said hammers from excessive current flow as in claim 2, wherein said means for preventing uncoupling of said power supply includes a gate which couples the output of said comparator to said means for decoupling said power supply from said hammers, said gate having an enabling input when said printer is not printing. 

1. In a high-speed impact printer in which a plurality of hammers can fire simultaneously, apparatus for protecting said hammers from excessive current flow comprising, in combination: a comparator having a signal input means, said comparator producing a characteristic output signal if the signal at said input means exceeds a predetermined level, means for generating a signal level proportional to the current flow respectively in each of said hammers, means for coupling said comparator input means to each of said generating means, a power supply, means for coupling said power supply to said print hammers to energize said print hammers, said power supply coupling means including means For decoupling said power supply from said print hammers in response to said characteristic output signal, said predetermined signal level being substantially less than the signal level generated by said generating means when a print hammer is energized, and means preventing said uncoupling of said power supply from said print hammers during that portion of a print cycle during which said print hammers are firing.
 2. In a high-speed impact printer in which a plurality of hammers can fire simultaneously, apparatus for protecting said hammers from excessive current flow as in claim 1, wherein each of said generating means includes a low-impedance resistor coupled in series with a hammer solenoid-operating coil and each of said means for coupling said generating means to said comparator input means includes a diode.
 3. In a high-speed impact printer in which a plurality of hammers can fire simultaneously, apparatus for protecting said hammers from excessive current flow as in claim 2, wherein said means for preventing uncoupling of said power supply includes a gate which couples the output of said comparator to said means for decoupling said power supply from said hammers, said gate having an enabling input when said printer is not printing. 